In the semiconductor industry, semiconductor materials such as silicon are hermetically sealed in ceramic packages. The function of the package is to provide a protective container for the silicon device.
In the past relatively high temperature sealing glasses of the lead zinc borate type have been employed in packaging. The more successful sealing glasses have been based on PbO:ZnO:B.sub.2 O.sub.3 in which the components are present at 2:1:1 mol ratios. These lead zinc borate glasses have been modified by the addition of minor amounts of other materials such as silicon dioxide, barium oxide and tin oxide. In the course of use glasses of this type have been thermally devitrified to reduce their thermal expansion. During devitrification the glass is converted from a solid solution to a rigid crystalline skeleton associated with a vitreous matrix characterized by reduced thermal expansion. Thermal expansion can be further reduced and the mechanical strength of the glass can be further increased by the addition of a small amount of a low expansion inert ceramic powder.
The devitrification may be carried out by maintaining the glass at a temperature about 430.degree. C. for a period of about an hour or longer. While these sealing conditions are acceptable in some environments, in the production of ceramic packages long exposure to high temperatures must be avoided and in order to obtain the high glass fluidity required for a proper seal without long high temperature exposure the glasses are commonly subjected to temperatures in the range of 480.degree. to 530.degree. C. for 8 to 12 minutes to complete the seal.
The requirement of a devitrification step during the sealing of ceramic packages with the glasses above described makes the employment of times and temperatures during the sealing process of the order of those described above necessary.
It is an object of this invention to provide sealing glasses which do not require devitrification to form a strong and hermetic seal.
It is a further object of this invention to provide sealing glasses which provide effective seals at greatly reduced time-temperature conditions.